Bridges of Washington State: A Slideshow Primer of Technology Through Time

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Trestle bridges have spans held up by individual towers. The towers are composed of two or more bents (vertical supports) braced together by cross pieces.

Often in old trestle bridges the bents lean toward each other. The towers are anchored into the ground with masonry, and the deck is then laid across. In the past they were made of wood. Notice the open work -- which lets wind pass through. Many early railroad bridges were trestle bridges.

Beam Bridge

The simplest form of a beam bridge is a log or flat stone laid across a stream. Its origin is in prehistoric times.

Today the most common type of beam bridge is a box-steel girder bridge. A girder is a supporting beam. In a beam bridge it is a horizontal beam supported at each end by a pier. A box steel girder is a steel beam shaped like a square drinking straw with a squared hollow running the long way through the center. Box construction is a way to lighten up a steel beam that would be much heavier if it were solid steel.

Truss Bridges, 3

Many truss bridges have more than one kind of truss. Bremerton's Manette Bridge has a deck truss and a through truss.

The deck truss is the one underneath the roadway. When you drive across the bridge you can't see it.

The through truss is the one on top. This truss type is called a Parker truss.

The Parker truss is a modification of the Pratt truss. It is a Pratt truss (the kind with the N configuration) with a polygonal top chord. That is, each strut is riveted to the next at an angle so that the top of the truss looks curved.

Truss Bridges, 1

A truss is a tinker-toy-like supporting structure made of "members" or struts (the straight pieces). The struts were formerly connected with pins. Later they were riveted. Now they are now bolted together. The original trusses were made of wood. Now they are made of steel (and rarely, concrete).

Trusses are placed in the configuration of triangles and are in tension and compression.

The truss bridge was the most common bridge form in Washington between 1880 and 1940, for both railroad and highway bridges. The truss has an open structure so that wind can blow through it instead of blowing against it and knocking it over.

A truss can be a "deck truss," in which the supporting truss is under the roadway. When you drive over a bridge supported by a deck truss, you can't see the truss at all. The Deception Pass Bridge, one of two bridges connecting Whidbey Island to Fidalgo Island, has a deck truss.

A truss can be a "through truss," the kind you drive through and under as you cross the bridge. The Lewis and Clark Bridge spanning the Columbia at Longview has a through truss. So does the Red Bridge on the Mountain Loop Highway in Snohomish County.

A truss can be a "pony truss," wihch rises on both sides of the roadway but is not cross-braced overhead. The Barstow Bridge in Stevens County has a pony truss.

Truss Bridges, 2

Trusses also have various configurations. The N shaped truss (simple verticals and diagonals) is called a Pratt truss, It was patented in 1844. The Barstow Bridge in Stevens County (now demolished) had a Pratt Truss.

The Warren truss has truss members configured like the letter W. It was patented in 1848 and is the most common truss in use today. The Aurora Bridge in Seattle has a Warren truss.

Arch Bridges

The arch bridge is an ancient type. The Romans constructed arch bridges out of stone. Arch bridges can be made of stone, steel, or concrete. These days concrete is the construction material of choice.

The Monroe Street Bridge, completed in Spokane in 1911, was at the time hailed as the largest concrete arch in the United States. It was built in the days before reinforced concrete.

The double-arched Fred Redmon (Selah Creek) Bridge was at the time it was built in 1971 the longest concrete arch in the United States. It is located near Yakima.

Moveable Bridges

There are several ways to move the bridge out of the way when a boat too big to clear the bridge needs to pass under.

The earliest moveable bridges in Washington were swing bridges. The swing bridge was a steel truss that rotated around a center pier. It could be rotated to be parallel with the river so that a boat could pass by.

The old swing bridges were mostly replaced with bascule bridges (or drawbridges). The first bascule bridges spanned moats leading to castles in the Middle Ages in Europe. The bascule bridge has one or two leaves that can lift. Each leaf has a counterbalance, a heavy weight that dips into the bascule pit when the bridge is opened.

A lift bridge has a center section that rises like an elevator to let a vessel pass underneath. The Murray Morgan Bridge in Tacoma is a vertical lift bridge.

Suspension Bridge, 1

Suspension bridges consist of huge towers, huge anchorages on each side of the bridge, and cables that run from one tower to the other, rather like a clothesline.

Hanging off the cables are steel ropes, vertical pieces called suspenders. The deck hangs from the suspenders.

Underneath the deck is a truss called the stiffening truss. It's purpose is to keep the deck from dipping according to whatever weight it is bearing. Without the stiffening truss, which is not attached to the towers, the bridge would act like a tightrope walkers rope, rising and falling according to the weight bearing down.

Suspension Bridge, 2

The huge cables on a suspension bridge cross over the towers and then run down the other side. On top of the tower the cable rests in what is called the saddle.

In this photo of one of the saddles of the new (2007) Tacoma Narrows Bridge, the cable is the thick green thing that looks like a curved pipe. The double ropes above it are handrails, used by workers who walk the cable.

Suspension Bridge, 3

To maintain the bridge, bridge workers walk up and down the cables. These days the workers are harnessed. They hold onto two handrails that run above the cable. This worker is walking the cable on the old Tacoma Narrows Bridge. Below you can see the crowd on the new (2007) Tacoma Narrows Bridge. It is opening day for the new bridge, July 15, 2007.

Cantilever Bridge

The cantilever bridge is a development of the beam bridge. Centilevers are arms extending from the anchor spans, one on each side. Some of these bridges have a center span hung between the two cantilever spans.

The Lyons Ferry Bridge that carries State Route 261 across the Snake River in the vacinity of Starbuck is an example of a cantilever bridge.

The bridge that crosses the Columbia at Wenatchee, opened in 1908 and now (2011) used as a footbridge, is the oldest extant steel cantilever bridge in the state of Washington.

Floating Bridges

Floating bridges float on huge, hollow, concrete pontoons. The pontoons are heavy but they are lighter than water.

Hybrid Bridges

As you can see from the bridges shown in this slideshow, many bridges combine different types. For example, most suspension bridges have a supporting truss under the deck. A floating bridge can have a non-floating part. A cantilever bridge can also be a truss bridge.

For example, the Aurora Bridge (formal name, George Washington Memorial Bridge), which crosses Lake Union in Seattle, is a cantilever bridge. It has a deck truss.

Cable-Stayed Bridges

Another form of suspension bridge is the cable-stayed bridge. The deck hangs on the cables themselves, which are suspended from the towers, making a typical A shape.

Notable Washington Bridges

Seattle's Dearborn Street Bridge, constructed by the City of Seattle in 1911, is the oldest extant steel arch bridge in the state.

Notable Washington Bridges

The first highway bridge to cross the Columbia River connects Wenatchee and East Wenatchee. Built in 1908, it is a 1,060-foot pinconnected steel cantilever bridge. The bridge carried pedestrians, horses, water, and later autos. Today (2011) it is a pedestrian bridge. It is the oldest extant steel cantilever bridge in the state.

Notable Washington Bridges

The Indian Timothy Memorial Bridge carries U.S. 12 over Alpowa Creek, in Asotin County not far from the Snake River. Built in 1923, it is a concrete-arch bridge build in a "rainbow arch" design made popular in the 1910s and 1920s by James Marsh, an engineer from Des Moines, Iowa.

Notable Washington Bridges

The Deception Pass Bridge is a steel cantilever bridge. Its fraternal twin, the Canoe Pass Bridge, is a steel arch bridge. They were financed by federal New Deal agencies and dedicated on July 31, 1935.

The new bridge faithfully replicates the venerable third bridge to stand on the site, which at the time of its opening in 1911, was the longest concrete-arch bridge in the United States. The reconstruction, with the same graceful arches, remains a marvel of beauty and functionality. The reconstruction retains the aesthetic elements of the earlier bridge, while improving strength and safety for a long future of heavy use.

Notable Washington Bridges

The new Tacoma Narrows Bridge was dedicated on July 15, 2007. It is a fraternal twin to the second Tacoma Narrows Bridge (1950), dubbed "sturdy Gerdie" to compare it to the first Tacoma Narrows Bridge, dedicated on July 1, 1940, collapsed on November 7, 1940.

This photo shows both bridges on the opening day of the new Tacoma Narrows Bridge. The new bridge, on the right, is filled with pedestrians, the only time this will ever occur.

In Washington, bridges are ubiquitous. As of August 4, 2010, there were 9,415 bridges on the Washington State Department of Transportation (WSDOT) inventory. These include all bridges 10 feet and longer, all bridges owned by state and local agencies, and docks and transfer facilities owned by Washington State Ferries. The inventory includes only a few federally owned bridges and it only includes railroad bridges that cross public roadways. Bridges are added and removed from the inventory every year at the rate of about 100 a year. Forest roads and hiking trails also incorporate bridges. This slideshow offers a brief overview of Washington bridges and bridge technology as it evolved over time.

Bridge design is always evolving. Even so, there are only a few basic types:

trestle bridge -- the span is supported by a row of simple towers with vertical supports (called bents) leaning toward each other and steadied by cross pieces.

beam bridge -- the simplest type is a log placed across a stream

cantilever -- a development of the beam bridge. You have two anchor spans, supported by piers. Then two arms, the cantilever spans, extend out from the anchor spans. Some of these bridges have a center span hung between the two cantilever spans.

truss bridge -- the supporting truss looks like a Tinker Toy structure: The connecting members or struts (straight pieces) come in different arrangements, such as triangles or an N shape.

moveable bridges -- these include swing bridges, lift bridges, and bascule bridges (or drawbridges) whose spans are raised and lowered like a see saw.

suspension bridge -- the deck is supported by cables suspended from towers. The cables run from tower to tower, and the deck is hung from vertical suspenders that hang from the cables.

cable-stayed bridge -- A different type of suspension bridge. The cables run from towers directly to the deck, forming an A shape.

floating bridge or pontoon bridge -- the original floating bridges may have been canoes tied together to reach across a river.

There are a few terms and concepts that bridge engineers and bridge aficianados and buffs commonly use.

The dead load is the weight of the bridge itself.

The live load is the weight of the traffic on the bridge, whether autos or pedestrians or sheep.

Bridges work by tension (pulling apart) and compression (pushing together). Take a rope or a towel in your two hands and pull it apart as hard as you can. That is tension. Put your hands together as if you are praying and push them together as hard as you can. That is compression. The struts in trusses are in tension and compression.

As you walk, bicycle, drive, or take the train whether in your home town or on a trip, being able to recognize bridge types and understand something of how they are put together will increase your enjoyment of these monumental icons of our built environment. All bridges must stand up and carry their load. Some are ordinary, even homely. Others in their grace and great beauty attain something approaching a work of art.

Cable passing through the saddle (manufactured by Atlas Castings and Technology), top of new (2007) Tacoma Narrows Bridge tower, shot from tower of 1950 Tacoma Narrows Bridge and showing handrails above the cable, Tacoma, July 15, 2007

Photo by M. Anne Sweet

New (2007) Tacoma Narrows Bridge (right, with pedestrians) and old (1950) Tacoma Narrows Bridge, shot from tower of old bridge, July 15, 2007

Photo by M. Anne Sweet

New (2007) Tacoma Narrows Bridge (right, with pedestrians) and old (1950) Tacoma Narrows Bridge, shot from tower of old bridge, July 15, 2007

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